1. Field of the Invention
The present invention relates to a method for producing an optical information recording medium of capable of add-on type recording and reproduction with a laser light of a specified wavelength.
2. Description of the Related Art
An optical information recording medium (optical disc) capable of information recording only once with a laser light is already known. Such an optical disc is also called an add-on CD (CD-R), and has a representative structure including, on a transparent disc-shaped substrate, a recording layer composed of by an organic dye, a light reflecting layer formed by a metal such as gold (metal reflective layer) and furthermore, a resinous cover layer (cover layer) laminated successively in this order. The recording of information on such a CD-R is executed by irradiating the CD-R with a laser light of a near infrared region (normally a laser light with a wavelength of approximately 780 nm), whereupon the irradiated part of the recording layer shows a local temperature elevation by light absorption to cause a physical or chemical change (e.g., a pit formation) thereby altering the optical characteristics of such part and thus recording the information. On the other hand, reading (reproduction) of the information is also executed by irradiating the CD-R with a laser light of the same wavelength same as that of the recording laser light, thereby detecting a difference in the reflectance between a part of the recording layer where the optical characteristics are changed (recorded part) and a part where the optical characteristics are not changed (unrecorded part).
Recently the need for optical information recording media of a higher recording density has increased. In order to meet the need, an optical disc known as add-on digital versatile disc (DVD-R) has been proposed (described, for example, in “Nikkei New Media”, separate edition “DVD”, published 1995). The DVD-R has a structure in which two discs are mutually adhered with recording layers positioned inside, each disc ordinarily including a recording layer containing an organic dye, a light reflecting layer (metallic reflective layer) and a protective layer, laminated in succession in this order on a transparent disc-shaped substrate bearing guide grooves (pre-grooves) for tracking an irradiating laser light, with a groove width (0.74 to 0.8 μm) equal to or less than a half of the groove width in a CD-R, or a structure in which such disc is adhered with a disc-shaped protective substrate of a same shape with the recording layer positioned inside. On a DVD-R thus structured, recording and reproduction of information are executed by irradiation of a visible laser light (having a wavelength ordinarily within a range of 630 to 680 nm), and the recording can be achieved with a recording density higher than that of the CD-R.
Recently, networks such as internet and high-vision television are rapidly increasing in popularity. Also the start of broadcasting of High Definition Television (HDTV) is forthcoming. Hence, there is a demand for a recording medium with a large capacity, capable of easily and inexpensively recording image information. Though the DVD-R is sufficiently serving as the large-capacity recording medium at present, the demand for a higher capacity and a higher density is ever-increasing, and the development of a recording medium capable of meeting those demands is necessary. For this reason, developments are being made for a recording medium with a larger capacity, capable of high-density recording with a light of a shorter wavelength than that of the DVD-R.
For example, JP-A Nos. 4-74690, 7-304256, 7-304257, 8-127174, 11-53758, 11-334204, 11-334205, 11-334206, 11-334207, 2000-43423, 2000-108513, 2000-113504, 2000-149320, 2000-158818 and 2000-228028 all disclose recording and reproducing methods for recording and reproducing information by irradiating an optical information recording medium, including a recording layer containing an organic dye, with a laser light of a wavelength not exceeding 530 nm in a direction from the recording layer toward a metallic reflective layer. In these methods, recording and reproduction of information are achieved by irradiating an optical disc, including a recording layer, which contains a porphiline compound, an azo dye, a metal azo dye, a quinophthalone dye, a trimethinecyanine dye, a dye of a dicyanobiphenyl skeleton, a coumarin compound, a naphthalocyanine compound or the like, with a laser light of blue color (wavelength 430 or 488 nm) or blue-green color (wavelength 515 nm).
On the other hand, an optical disc of phase change type is known as a DVD, which is based on a system of employing a layer of an alloy such as GeSbTe for the recording layer, instantaneously heating the recording layer with a laser light to cause a phase change from a crystalline state to an amorphous state and utilizing a change in the reflectance, caused by the phase change, for information recording and reproduction. A DVR system employing a DVD of such phase change type and executing recording and reproduction with a blue-violet laser was recently announced (“ISOM2000”, pp. 210–211). This system achieves a certain result for the target toward a higher density.
In an optical information recording medium to be employed in such an optical recording system utilizing a blue-violet laser and a pickup of a high numerical aperture (NA), in order to focus an objective lens of a high NA at the irradiation of the recording layer with a blue-violet laser light, it is preferred to use a thin cover layer through which the laser light enters. The cover layer is formed for example by a thin film, which is adhered to the recording layer with an adhesive material or a sticking adhesive material. A thickness of the cover layer, including an adhesive layer formed by the hardened adhesive material, is usually about 100 μm, but is optimized according to the wavelength and the NA of the irradiating laser light.
Such an optical disc at least includes a recording layer and a cover layer on a disc-shaped substrate, and, during manufacture, is subjected to an adhering step in which a cover sheet is adhered with an adhesive material such as a UV curable adhesive or a sticking adhesive material on a surface, bearing at least the recording layer, of the disc-shaped substrate, thereby forming a cover layer. However, such adhering step is susceptible to formation of a fault portion by causing an inclusion of dust in the adhesive layer, formed by adhering the above-mentioned surface bearing the recording layer and the above-mentioned cover sheet, thus resulting in a dropout in the optical disc, namely in a loss in the production yield.
Also, JP-A No. 2000-67468 discloses a method of forming the cover layer by continuously forming a laminated film member of a configuration in which a laminate member formed by a transparent film and a sticking adhesive sheet is sandwiched on both surfaces between releasing films, then working the laminated film member into a disc shape to obtain a disc-shaped laminated film member, and adhering the laminated film member to the recording layer-bearing surface of optical disk substrate with a positional alignment, while peeling off the releasing film of a side in contact with the sticking adhesive sheet.
With such a method, in case the laminated film member after the continuous formation thereof is wound in a roll, the laminated film member working into the disc shape tends to show a bending by such rolling or a curl, thereby leading to a cover layer having an unflat surface or an unevenness in the thickness. As a result, an optical information recording medium having such a cover layer is unable to achieve satisfactory recording characteristics or a satisfactory durability.
Further, in the manufacturing process of the above-described optical information recording media, in the case of a DVD-R or a CD-R, there is a generally adopted procedure of forming a recording layer on a substrate and then forming a metal reflective layer thereon. Therefore, the surface of the metal reflective layer at the side of the recording layer is not exposed to the air. On the other hand, with a DVR, a procedure has been adopted of forming a metal reflective layer on a substrate and then forming a recording layer thereon, so that a surface of the metal reflective layer at the side of the recording layer is exposed to the air until the formation of the recording layer. The metal reflective layer is generally formed by sputtering a metal such as Ag or Al, and, if thus formed layer is exposed to the air, the surface thereof is oxidized by the moisture in the air or by the influence of temperature, whereby the surface roughness of the metal reflective layer is deteriorated.